The present invention pertains to methods and apparatus for maintaining the temperature of an area or apparatus at a predetermined value. More particularly, the present invention relates to techniques for stabilizing the temperature of heat-sensitive equipment, such as radiation detectors, at sufficiently low temperatures to enhance the performance of such equipment.
Monitoring and measuring techniques employing the activation of materials under investigation, and the detection of resulting radiation find increasing use in the oil industry. One such application involves the monitoring of crude oil flowing in a line between an oil well and a storage tank or other destination. In such an application, small amounts of salt in the crude oil may be detected by utilizing the characteristic gamma radiation emitted by the chlorine in the salt when the chlorine is first activated. A neutron source is placed within the pipeline to radiate the chlorine. A continuous chemical neutron source, such as an actinium berylium source, or a californium 252 source may be used. The radioactive isotope chlorine 36 is generated according to the reaction Cl.sup.35 (n,.gamma.)Cl.sup.36. Then the unstable chlorine 36 isotope decays by the emission of gamma radiation predominantly in the energy range of 5.5 to 8.0 MEV. The gamma rays may be detected by an appropriate radiation detector placed a short distance downstream within the pipeline.
The radiation detector may be a sodium iodide thallium-activated scintillation crystal. The crystal is optically coupled to a photomultiplier tube. As is well known, gamma rays entering such a crystal interact therewith to produce light falshes, or scintillations, whose intensity is functionally related to the gamma ray energy. The light flashes are then detected by the photomultiplier tube to generate voltage pulses proportional in magnitude, or height, to the intensity of the corresponding light flashes. Thus, a succession of pulses is produced by the photomultiplier tube, wherein each pulse is proportional to the energy of the corresponding incident gamma ray. The pulse stream from the photomultiplier tube is received by an amplifier which amplifies the pulse signal before it is transmitted to appropriate data processing equipment used to analyze the pulse data to acquire information concerning the oil flowing in the pipeline.
In order to function optimally, the scintillation crystal, photomultiplier tube and amplifier must operate at a constant temperature, preferably below room temperature. The photomultiplier tube is particularly sensitive to temperature variations, with the signal-to-noise ratio decreasing as the temperature of the photomultiplier tube increases. Thus, when the detector assembly is exposed to a high temperature environment, such as within a crude oil pipeline, wherein the temperature may be as high as 90.degree. C., a means must be provided to cool and stabilize the detector assembly at or below room temperature.
The detector assembly may be cooled by thermal contact with a coolant. In particular, a phase change material, such as ice or other chemical, may be used to absorb heat, transmitted to the detector assembly, at the phase change temperature. However, such a cooling system is time-dependent, and limited by the volume of phase change material available for thermal communication with the detector assembly.